Slitless spectroscopy is astronomical spectroscopy done without a slit to allow only light from a large region to be diffracted. Slitless spectroscopy works best in sparsely populated fields, as it spreads each point source out into its spectrum, and crowded fields will be too confused to be useful. Slitless spectroscopy also faces the problem that for extended sources, nearby emission lines will overlap. Grisms are often used in slitless spectroscopy. Grisms are a combination of gratings and prisms arranged so that light at a chosen central wavelength passes straight along the optical axis. The advantage of a grism is that all optical components can be placed along a straight line. However, when using a grism, and in spectrometers in general, there can be a problem with capturing images, necessitating two separate cameras to be used, one for imaging (without the grism) and one for spectroscopy (with the grism). The grism is inserted into a camera beam that is already collimated or produced from a large focal ratio. The grism then creates a dispersed spectrum centered on the object's location in the camera's field of view.
While prior art gratings and grisms systems have been used by a number of astronomical telescopes to perform slitless spectroscopy, the systems have many shortcomings. For example, the Rspec Star Analyser (RSA) is a simple diffraction grating housed in a 1.25″ filter cell. The RSA is designed to be mounted into an eyepiece or filter slot for an astronomical camera. However, the RSA only comprises a simple grating without prisms or collimating optics. Because of this, the RSA suffers from focal plane curvature which prevents the entire visible spectrum to be focused during a single exposure. In addition, the RSA employs a very low resolution grating (e.g., 100 to 200 lines per millimeter) to minimize the effects of focal plan curvature. The use of a low resolution creates a drawback, namely that closely spaced spectral lines become blended together at low resolution. Also, for most astronomical sources, the spectral line width will be smaller than the resolution provided by the RSA. This results in blending the line with neighboring continuum, and thus a lower signal sensitivity to weak, narrow features.
In addition, existing compact spectrographic systems lack the correcting optics necessary to produce a focused spectrum over the entire visible spectrum. For example, the Hubble Space Telescope (HST) also utilizes grisms in order to perform slit-less spectroscopy, but only in both the ultra-violet and the infrared wavelengths using a Wide Field Camera 3 (WFC3) instrument. These grisms do not operate between 450 nm and 800 nm, leaving the majority of the visible spectrum un-observed. Additionally, the grisms installed within WFC3 all have low spectral resolutions. In some instances, these grisms can be better for seeing the overall shape of the continuum emissions since the light is not spread out as much over the detector. However, high spectral resolution is better for observing the narrow spectral lines. Further, the HST has a large focal ratio of f/24. Because of this, the light passing through the grism is approximately parallel and the degradation of the focus is negligible. However, many other telescopes have a much smaller focal ratio, with which a simple grism would be unable to focus.
Further, while some instruments do utilize internal collimating optics to correct for focus problems, these systems utilize large external collimators unsuitable for small telescopes. For example, the Very Large Telescope (VLT) utilizes the Focal Reducer and low dispersion Spectrograph (FORS). The FORS instrument possesses collimating/focal reducing optics. However, the FORS collimator is located before the filter wheel and must be used for all imaging filters and grisms. The design of the FORS instrument allows it to be used on a telescope with a small focal ratio. But the FORS is extremely large (several feet long) and is very heavy. An instrument of a similar design to FORS would be too large to install on amateur or university class telescopes. Additionally, FORS is a custom instrument designed specifically for the VLT, leading to an extremely high cost.
Last, current moderate to high resolution spectrometer systems for small telescopes require the user to remove the imaging camera to install the spectrometer, preventing the user from easily switch between imaging and spectroscopy. For remote controlled or robotic telescopes, users must dedicate their telescope to either spectroscopy or imaging, and cannot switch between the two without sending someone to the remote site to change the instrument.
Therefore, there is a need for a low cost grism system that includes a high resolution grating that can disperse visible light using optical telescopes with small focal ratios. In addition, there is a need for a system that allows users to quickly switch between imaging and spectroscopy, without the need of removing or installing equipment on the telescope.